The present invention relates to a method of driving a solid-state image pickup device for transferring and reading signal charges achieved in an image pickup area line by line by a horizontal shift register.
In an image pickup device for generating an image signal by using a solid-state image pickup device or the like, images can be enlarged by not only optical zooming using a lens (optical zoom), but also zooming based on signal processing (digital zoom). Particularly, in the case of an image pickup device using a solid-state image pickup device, in connection with increase in number of pixels of the solid-state image pickup device, sufficient resolution can be achieved even when digital zooming is carried out.
Furthermore, in a solid-state image pickup device using a CCD shift register as means of reading signal charges, signal charges of all the pixels of an image pickup portion are read out as an image signal, and a portion of the image signal which corresponds to an enlargement target area is subjected to the signal processing to perform digital zooming.
Here, a method of driving a solid-state image pickup device for a conventional digital zooming operation will be described by using a frame transfer CCD image sensor as a solid-state image pickup device.
FIG. 1 is a schematic plan view showing a frame transfer CCD image sensor. The image sensor 2 comprises an image pickup portion 2i, a charge storage portion 2s, a horizontal transfer portion 2h and an output portion 2d. Each of the image pickup portion 2i and the charge storage portion 2s comprises plural vertical CCD shift registers. Signal charges accumulated in respective cells of the image pickup portion 2i by light exposure are frame-transferred to the storage portion 2s by the vertical transfer operation of the vertical CCD shift registers. The horizontal transfer portion 2h comprises a CCD shift register, and each bit of the horizontal transfer portion 2h is connected to the output of each of the plural vertical CCD shift registers of the storage portion 2s. The signal charges of one frame held in the storage portion 2s are transferred to the horizontal transfer portion 2h line by line. The horizontal transfer portion 2h successively transfers the signal charges of each line to the output portion 2d. The output portion 2d receives the signal charges output from the horizontal transfer portion 2h one bit at a time by using capacitance to convert the signal charges thus received to a voltage, and outputs the voltage thus converted as a time-sequential image signal.
According to the conventional driving method, signal charges of an n-th line which are transferred to the respective bits of the horizontal transfer portion 2h are discharged to the output portion 2d in the horizontal transfer mode by the amount corresponding to all the bits, and then signal charges of (n+1)-th line are transferred from the storage portion 2s to the horizontal transfer portion 2h. In the prior art, an image signal is generated from the image sensor 2 by reading out the signal charges of all the cells which are achieved by the image pickup portion 2i. When an image in a partial area of the image pickup portion 2i is used for digital zooming or the like, a part of the image signal output from the image sensor 2, corresponding to the partial area concerned, is cut out from the image signal in a signal processing circuit disposed at the rear stage of the output portion 2d to generate an image signal corresponding to an enlarges image.
The processing of extracting a partial image of the whole image in the image pickup portion of the solid-state image pickup device can be used for the digital zooming as described above.
Here, in digital cameras and other portable terminals having a camera function, the number of pixels of a solid-state image pickup device is more remarkably enhanced than the number of pixels of a monitor for preview. Therefore, there is a case where when an image is recorded in a recording medium such as a memory or the like, an image is picked up with high resolution corresponding to the number of pixels of a solid-state image pickup device, however, it is sufficient to pickup an image with a small number of pixels which corresponds to the number of pixels of a preview monitor. Furthermore, from the viewpoint of a human visual characteristic, the motion picture does not need resolution which is as high as the still picture, and there is a case where image pickup based on a smaller number of pixels than that of a still picture is selectively carried out in order to reduce the amount of recording data. In these cases, the processing of extracting an image from a part of the image pickup portion of the solid-state image pickup device and using the image thus extracted may be carried out.
In the case of such a situation that only an image of an image extraction target area corresponding to a part of the image pickup portion is used as described above, it has been hitherto general that all the signal charges of all the cells of the CCD image sensor are read out line by line as described above. That is, there has been hitherto such a problem that when an image of a partial area is extracted, signal charges of unnecessary cells must be successively read out through the horizontal transfer portion and it is difficult to keep a sufficient frame rate when a preview is made or motion pictures are picked up. Furthermore, unnecessary cells other than the cells of the image extraction target area must be read out, and thus the number of driving clocks of the horizontal transfer portion is increased by the amount corresponding to the read-out of the unnecessary cells, so that the power is wastefully consumed. Here, if the transfer clock frequency of the horizontal transfer portion is increased, it would be possible in principle to increase the frame rate. However, actually, the transfer efficiency in the horizontal transfer is lowered and thus increase in transfer clock frequency is limited. When a still picture is picked up, if several frames at maximum can be picked up per second, it would be sufficient. Therefore, in this case, it is little required to increase the frame rate. Accordingly, in the case of the image pickup operation of still pictures, it is possible to secure a relatively-long horizontal transfer period even when all the area of the image pickup portion is read out.
As described above, it has been hitherto difficult to increase a frame rate when only an image of an image extraction target area set in a part of the image pickup portion of the solid-state image pickup device is used.